The present invention relates to the color imaging arts. It finds particular application to color correction tables for printers and will be described with particular reference thereto.
Heretofore, computers and other electronic equipment have typically generated three-dimensional RGB (red, green, blue) color signals. Many printers, however, receive four-dimensional CMYK (cyan, magenta, yellow, and black) signals as input and print output colors which are measured as corresponding RGB values. A look-up table is commonly provided to convert each digital RGB color signal value to a corresponding digital CMYK value before being received by the printer.
A printer which has an ideal dye behavior has a one-to-one correspondence of cyan-to-red, magenta-to-green, and yellow-to-blue. This means that when printed, the cyan ink will only absorb red light, the magenta ink will only absorb green light, and the yellow ink will only absorb blue light. However, printers inherently have a non-ideal dye behavior and therefore have a complex non-linear colorimetric response. Interactions between the cyan, magenta, and yellow inks exist which result in unwanted absorptions of reds, greens, and blues. Even once a printer is calibrated such that one or a range of input digital CMYK values produce the proper color(s), the full spectrum of CMYK values and printed colors is not accurate. In other words, the colors asked to be printed and the actual colors printed are not the same.
This discrepancy arises because the relationship between digital values that drive the printer and the resulting colorimetric response is a complex non-linear function. A response, or other value, labeled as "colorimetric" indicates that the response or value which has been measured by an instrument. Modeling the colorimetric response to achieve linearity across the available spectrum usually requires many parameters. Therefore, the relationship between the CMYK values driving the printer and the measured colorimetric RGB values of the resulting printed patch is often not characterizable by a simple function or model. The number of measurements required to characterize the printer adequately, can be as many as 1,000 measurements. Typically, a color correction look-up table is built which approximates the mapping between RGB colorimetric space and CMYK values. More specifically, the color correction look-up table corrects for non-linearities and unwanted absorptions of inks such that the printer prints the true corresponding color.
To build the look-up table, a set of CMYK digital values are sent to the printer. The printer prints a corresponding set of color patches. The color patches are measured and a colorimetric RGB coordinate is found for each patch, hence for each CMYK value. Each of the RGB coordinates identify a three-dimensional vector location within the three-dimensional space. Each RGB coordinate is typically represented by an 8-bit red value, an 8-bit green value, and an 8-bit blue value. Although the RGB coordinate is capable of addressing 256.sup.3 locations, the look-up table is typically partitioned into a smaller size, such as 16.times.16.times.16 (4096) table locations, each of which stores a four-dimensional CMYK value. The number of table locations is selected based on the desired accuracy of the look-up table compared to the expense of storing a large number of values.
At each measured RGB location, the CMYK value corresponding to the RGB coordinate is known. Measured RGB coordinates do not in general coincide with the node locations (intersection points) of the look-up table. Hence, the CMYK values to be filled-in at the table nodes are estimated from the known CMYK values at the measured RGB locations by an interpolation technique such as Shepard's algorithm. Once the table is built, RGB coordinates are converted to CMYK values by interpolating the nearest known CMYK values neighboring the inputted RGB coordinate location to obtain an interpolated printer CMYK value. The interpolating is commonly a tetrahedral or trilinear interpolation. Thus, the variables in the design of the correction table are the RGB locations, the CMYK values at the locations, and the method used to interpolate between the CMYK values.
Due to drifts in printer response and the interpolation technique used to map the transformations, a particular colorimetric RGB location may not produce the correct printer CMYK value. In order to adjust for the difference, CMYK values are reprinted and the color patches are remeasured to determine the error. Remeasuring color patches is an expensive and time-consuming process.
The present invention provides a new and improved method and apparatus for refining a color correction table which overcomes the above-referenced problems and others.